1. Field of the Invention
The present invention relates to a control device for a four-wheel drive vehicle, comprising yaw moment control for generating yaw moment on the vehicle, driving force control for suppressing excess driving force, and limited slip differential control for limiting differential motion between the right and left wheels.
2. Description of the Related Art
Four-wheel drive vehicles manufactured in recent years are equipped with a number of vehicle behavior control devices such as yaw moment control for generating yaw moment on the vehicle, driving force control (traction control) for suppressing excess driving force, limited slip differential control (LSD control) for limiting differential motion between the right and left wheels, and the like. Since these controls often interfere with each other, a wide variety of techniques have been developed and utilized in order for these controls to effectively work together to their full potential.
For example, Japanese Laid-open Publication 2004-106649 discloses a technique wherein transfer clutch torque is adjusted to be small enough to avoid the interference of the behavior controls such as the yaw moment control, the traction control, or the ABS (Anti-lock Brake System) operation in a four-wheel drive vehicle where the driving force is distributed between front and rear axles by, instead of a center differential, a transfer clutch that can vary the transfer capacity. The entire disclosure of the aforesaid publication is incorporated herein by reference.
For example, when the yaw moment control operates to prevent an over-steering tendency, the braking force is applied to the front wheel located at the outer side of turning. In this case, the braking force due to the limited slip differential control is applied to the front wheel located at the inner side of the turning, because a difference in wheel velocity has been generated between the right and left front wheels. While the cornering force generated by the front tires is maintained owing to the braking forces applied to the two front wheels, the driving force is transferred to the two rear wheels. If the traction control via a switch means is effective, the engine control prevents the transfer of excess driving force to the two rear wheels. However, if the traction control is not effective, the driving force is excessively transferred to the two rear wheels so that the cornering force generated by the rear tires tends to decrease. As a result, the yaw moment along the same direction as the vehicle's yaw moment is generated because of the reduction in the cornering force at the two rear wheels while the cornering force at the two front wheels is being maintained. Thus, the over-steering tendency cannot be prevented after all. In this case, if a transfer clutch that can vary the transfer capacity is used for the driving force distribution instead of a center differential, as disclosed in the aforesaid Japanese Laid-open publication 2004-106649, it is possible to reduce the torque transfer from the front wheels to the rear wheels by reducing the transfer clutch torque, thereby preventing the interference of the yaw moment control with the rear wheels. However, in the center differential mechanism wherein the torque distribution between the front and rear axles is controlled by means of a clutch that can vary the transfer capacity, reducing the engagement torque of the clutch may permit the transfer of excess driving force to the rear wheels, likely to cause unstable vehicle behaviors.